[NETFILTER]: xt_TCPMSS: don't allow netfilter --setmss to increase mss

When terminating DSL connections for an assortment of random customers, I've
found it necessary to use iptables to clamp the MSS used for connections to
work around the various ICMP blackholes in the greater net.  Unfortunately,
the current behaviour in Linux is imperfect and actually make things worse,
so I'm proposing the following: increasing the MSS in a packet can never be
a good thing, so make --set-mss only lower the MSS in a packet.

Yes, I am aware of --clamp-mss-to-pmtu, but it doesn't work for outgoing
connections from clients (ie web traffic), as it only looks at the PMTU on
the destination route, not the source of the packet (the DSL interfaces in
question have a 1442 byte MTU while the destination ethernet interface is
1500 -- there are problematic hosts which use a 1300 byte MTU).  Reworking
that is probably a good idea at some point, but it's more work than this is.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/xt_TCPMSS.c

@@ -88,8 +88,11 @@ tcpmss_mangle_packet(struct sk_buff *skb,
 
 			oldmss = (opt[i+2] << 8) | opt[i+3];
 
-			if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
-			    oldmss <= newmss)
+			/* Never increase MSS, even when setting it, as
+			 * doing so results in problems for hosts that rely
+			 * on MSS being set correctly.
+			 */
+			if (oldmss <= newmss)
 				return 0;
 
 			opt[i+2] = (newmss & 0xff00) >> 8;